Re: [dpdk-dev] [PATCH 2/2] eal/x86: Use lock-prefixed instructions to reduce cost of rte_smp_mb()

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Bruce,

> -----Original Message-----
> From: Richardson, Bruce
> Sent: Monday, December 11, 2017 5:11 PM
> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 2/2] eal/x86: Use lock-prefixed instructions to reduce cost of rte_smp_mb()
> 
> On Fri, Dec 01, 2017 at 11:12:51AM +0000, Konstantin Ananyev wrote:
> > On x86 it  is possible to use lock-prefixed instructions to get
> > the similar effect as mfence.
> > As pointed by Java guys, on most modern HW that gives a better
> > performance than using mfence:
> > https://shipilev.net/blog/2014/on-the-fence-with-dependencies/
> > That patch adopts that technique for rte_smp_mb() implementation.
> > On BDW 2.2 mb_autotest on single lcore reports 2X cycle reduction,
> > i.e. from ~110 to ~55 cycles per operation.
> >
> > Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> > ---
> >  .../common/include/arch/x86/rte_atomic.h           | 45 +++++++++++++++++++++-
> >  1 file changed, 43 insertions(+), 2 deletions(-)
> >
> <snip>
> > + * As pointed by Java guys, that makes possible to use lock-prefixed
> > + * instructions to get the same effect as mfence and on most modern HW
> > + * that gives a better perfomarnce than using mfence:
> > + * https://shipilev.net/blog/2014/on-the-fence-with-dependencies/
> > + * So below we use that technique for rte_smp_mb() implementation.
> > + */
> > +
> > +#ifdef RTE_ARCH_I686
> > +#define	RTE_SP	RTE_STR(esp)
> > +#else
> > +#define	RTE_SP	RTE_STR(rsp)
> > +#endif
> > +
> > +#define RTE_MB_DUMMY_MEMP	"-128(%%" RTE_SP ")"
> > +
> > +static __rte_always_inline void
> > +rte_smp_mb(void)
> > +{
> > +	asm volatile("lock addl $0," RTE_MB_DUMMY_MEMP "; " ::: "memory");
> > +}
> 
> Rather than #defining RTE_SP and RTE_MB_DUMMY_MEMP, why not just put the
> #ifdef into the rte_smp_mb itself and have two asm volatile lines with
> hard-coded register names in them? It would be shorter and I think a lot
> easier to read.

Fine by me.
Any other thoughts from anyone till I submit v2?
Konstantin